Rearrangement of ethane‐1,2‐diol radical cations: Processes involving dipole‐catalyzed proton shifts and charge transfer

Abstract

Rearrangement and dissociation processes of solitary ethane-1,Zdiol radical cations were investigated by ab in& MO calculations, executed at the SDCI//RHF/DZP level of theory, including Pople-type size-consistency corrections. In order to obtain an accurate description of the chemistry involved, part of the potential energy surface was investigated by using the multi-reference CI method and also by using the valence bond (VB) method followed by SDCI calculations using the natural orbitals of the VB wavefunction. The ethane-l,2diol radical cation is metastable with respect to CH30HZ+ + HCO'; it has been shown recently that the isotopologue DOCH,CH,OD loses (exclusively) H C O to produce CH,DOHD+, not the isotopomer CH30D,+ expected from earlier mechanistic proposals. We have traced a low-energy pathway which explains the observed label distribution and which takes place at the experimentally derived energy level. First, ionized ethane-1,Zdiol collapses to the one-electron bond species [ HOCH, . . + *. . CH,OH I +. which subsequently rearranges to the hydrogen-bonded species CH,=O.*-HO(H)CH:'.Next, transformation to the transient CH,=O.*.HCH,OH+' takes effect and this rearrangement can be viewed as the 1,Zhydrogen shift, CH,OHi' + CH30H+', catalyzed by formaldehyde. Following this, charge transfer takes place from the methanol cation to the formaldehyde molecule which thus becomes charged; because it is now charged, the formaldehyde unit can rotate and donate a proton to the methanol molecule, after which dissociation follows. Our calculations and experimental results can be interpreted in terms of proton shifts rather than hydrogen shifts taking place in ion-molecule (proton-bound) complexes. Ab inilio calculations indicated that these intermediate ion-dipole complexes (hydrogen-bridged structures)enjoy considerable stability (they lie below ions CH20HCH20H+'), making them attractive intermediates. However, attempts to locate the corresponding transition state failed.5A different mechanistic proposal came from the Radom group.7 They showed from ab initio calculations that ethane-1,2-diol ions, 1, produced by vertical ionization (la) spontaneously undergo an intramolecular proton migration to produce the drstonic ion 'OCH2CH20H2+, 2, which was calculated to lie 69 kJ mol-below ions la. The adiabatic ionization energy (IE,) of ethane-1,2-diol' was proposed to correspond to the formation of structure 2.' The latter ions were proposed to dissociate via a 1,2-hydrogen shift to CH,OH,+ + HCO'. Note that both mechanisms

Anab InitioComputational Molecular Orbital Study of Radical, Protonated Radical (Radical Cation), and Carbocation Species That Have Been Proposed in Mechanisms for the Transfer Process in the Enzyme−Coenzyme B12-Catalyzed Dehydration of 1,2-Dihydroxyethane

“…Instead, we found that the initial structure for 4 transformed without activation to a hydrogen-bonded hydrate of the anti -vinyl alcohol radical cation, 19 , 27.2 kcal/mol lower in energy. Frequency analysis confirms that 19 is a local minimum on the C 2 H 6 O 2 ⎤•+ PES. 10a,d The transformation involves the H 2 O group on the right-hand carbon atom, C 2 , moving far to the left of C 1 without the formation of the protonated gem -diol radical structure 6 as an intermediate. In the optimized structure, C 2 ···O 4 = 4.540 Å and C 1 ···O 4 = 3.144 Å.…”

“…At level B , 20 is found to be 15.8 kcal/mol higher in energy than 19 , of which 2.3 kcal/mol can be attributed to the difference in stability between the syn - and anti -rotamers of the vinyl alcohol radical cation; see Table 21S. The hydrogen-bonded hydrate of the syn -vinyl alcohol radical cation, 21 , which results from a 180° rotation of the H 2 OHO− grouping about the C−O bond or inversion of the HO group with respect to the C−O bond in the anti -structure, 10a,d is considerably more stableonly 1.8 kcal/mol higher in energy than 19 at level A ; see Table 21S. − …”

An Experimental and Theoretical Study of the Long-Lived Radical Cation of CH3OCH2CH2OH

“…However, the C−C bond in the UMP2/6-31G** optimized structure is slightly shorter (1.775 Å). The presence of a long C−C bond in an organic radical cation is not unprecedented: the ions CH 3 CH 2 OH •+ and HOCH 2 CH 2 OH •+ have been calculated to have structures with remarkably long one-electron C−C bonds (CH 3 CH 2 OH •+ , 2.013 Å at the UHF/4-31G level; 40a,b HOCH 2 CH 2 OH •+ , 2.01 Å at the RHF/DZP level 40c ). The results obtained in the above calculations, especially the long C−C bond in 1‘‘ , as well as the location of the greatest positive charge at the hydroxyl hydrogen in this ion (Figure ), support the proposal that the entering nucleophile initially forms a hydrogen bond with the hydroxyl hydrogen, as shown in Scheme .…”

“…17 This ion was suggested to undergo a hydrogen atom shift to yield another hydrogen-bridged intermediate, (CH 3 ) 2 -O‚‚‚H‚‚‚OdCH •+ (3), which decomposes by expulsion of CHO • . A similar mechanism was proposed 19 for the loss of CHO • from the molecular ion of ethylene glycol, but this mechanism was later proved to be incorrect through labeling experiments. 20,21 The role of hydrogen-bridged intermediates in the dissociation of CH 3 OCH 2 CH 2 OH •+ was reinvestigated 20,21 by using differently deuterated analogues of CH 3 OCH 2 CH 2 OH.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Both types of ions have been established to represent stable gas-phase species. 1,4,6b,13 The 2-methoxyethanol radical cation CH 3 OCH 2 CH 2 OH •+ (1) undergoes interesting unimolecular dissociation reactions, [16][17][18][19][20][21][22] some of which have been proposed to involve hydrogen-bridged and/or distonic radical cation intermediates. The strong intramolecular hydrogen bond 18 in the neutral CH 3 OCH 2 CH 2 OH was initially thought to remain after ionization (1, Scheme 1) and to lead to isomerization of the initially formed radical cation to a hydrogen-bridged complex of formaldehyde and the methoxymethyl radical, CH 3 O(CH 2 )‚‚‚H‚‚‚OdCH 2 •+ (2, Scheme 1).…”